The present invention relates to a scanning electron microscope and, more particularly, to a scanning electron microscope using a retarding method in which an acceleration voltage of a primary electron beam is reduced just before being incident on a sample.
The basic construction of a conventional common scanning electron microscope will be described below, referring to FIG. 1. An electron beam 19 emitted from an electron source, not shown, toward a sample 9 is deflected by a scanning coil 2 so as to two-dimensionally scan on the sample 9. The electron beam 19 passed through the scanning coil 2 is converged to a fine beam by an objective lens 3 composed of a magnetic path 4 and an exciting coil 5, and irradiated to the sample 9. By the irradiation of the electron beam 19, secondary electrons 16 and reflected electrons 15 of the electron beam 19 are generated.
The secondary electrons 16 generated from the sample 9 are attracted since their energy is as small as 2 eV and accelerated by an electric field 11 of the scintillator 612 applied with a positive high voltage (10 kV) to make the scintillator 612 emit light . The generated light is transmitted in a light guide 613 and enters into a photomultiplier 614 to be amplified and converted into an electric signal.
An outer cylinder 610 is provided to make an effective distribution of the attractive electric field 11.
Since the electrons 15 reflected by the sample 9 have nearly the same energy as an energy (for example, 10 kV) at the time when the electron beam 19 is irradiated onto the sample 9, the electrons 15 travel in a direction opposite to the traveling direction of the electron beam 19 as shown in FIG. 1 without being attracted by the electric field 11 of the scintillator 612, and are incident on the scintillator 6 to make it emit light. The light is transmitted in the light guide 7 and enters the photomultiplier 8 to be converted into an electric signal.
An image of the sample shape can be displayed by performing brightness modulation using output intensities of the photomultipliers 8 or 614 and displaying the signals on a CRT with corresponding to scanning positions of the scanning coil 2 on the sample 9.
For shape inspection on a silicone wafer after being processed in the semiconductor industry, an optical microscope was used in the past, but a scanning electron microscope has been used in recent years because lines of a semiconductor device become smaller and resolution of an optical microscope is insufficient. Since a sample to be observed in the semiconductor industry is an insulator in most cases, it is required that energy (acceleration voltage) of an irradiated electron beam (primary electron beam) is set below 1 kV in order to prevent the sample from being charged by the irradiation of electrons.
However, when the acceleration voltage is decreased in the scanning electron microscope, the primary electron beam becomes difficult to be converged thin. Therefore, a retarding method has been practically used in recent years. In the retarding method, a primary electron beam of high acceleration voltage (for example, 2 kV) is converged to a thin beam by an objective lens and then the acceleration voltage of the primary electron beam is decreased just before being incident on a sample by a deceleration electric field (for example, -1 kV) applied between the objective lens and the sample. An example of the retarding method is described in FIG. 3 on page 402 of Ultramicroscopy 41 (1992) or SPIE Vol.2725 (1996), pages 105-113.
The retarding method has an advantage in that chromatic aberration caused by energy deviation of the electron beam can be lessened and the electron beam can be converged to a thin beam since the energy of the electron beam at the time when the electron beam is passing through the objective lens 3 can be set to a desired energy higher than the energy at the time when the electron beam is irradiated on the sample. On the other hand, the retarding method has a problem in that the secondary electrons is difficult to be detected because the secondary electrons generated from the sample are accelerated by the deceleration electric field between the objective lens and the sample and sucked inside the objective lens, and travel toward the electron beam source under lens action of the objective lens while being focusing.
The technology described in the above-mentioned paper, SPIE Vol.2725 (1996), pages 105-113, discloses a construction in which in order to detect the accelerated secondary electrons, a reflecting plate made of a metal is arranged at a position nearer to the electron beam source side than the objective lens, and the accelerated secondary electrons are collided onto the reflecting plate to further generate secondary electrons (called as signal secondary electrons) from the reflecting plate, and then the signal secondary electrons are deflected by an electric field to be detected by a detector.
In order to reduce the chromatic aberration of the objective lens, it is preferable that the acceleration voltage of the primary electron beam at the time when passing through the objective lens is set to a high voltage. In the retarding method, the difference between the acceleration voltage and the deceleration voltage of the primary electron beam is the energy of the electron beam when being incident on the sample. The following cases can be considered. For example, a case where a primary electron beam having an acceleration voltage of 5 kV is decelerated by a deceleration electric field of -4 kV and then is incident on a sample, or a case where a primary electron beam having an acceleration voltage higher that the above is decelerated to 1 kV and then is incident on a sample. In the retarding method, the secondary electrons from the sample are, however, accelerated by the deceleration electric field, as described above. Therefore, when the deceleration voltage is, for example, -4 kV, the secondary electrons are accelerated to 4 kV.
In order to efficiently detect the secondary electrons from the sample using the reflecting plate described in SPIE Vol.2725 (1996), pages 105-113, it is necessary to generate the signal secondary electrons by the reflecting plate with high efficiency. It is known that, in general, an efficiency of generating secondary electrons becomes a highest value (above 1) when an energy of the primary electrons incident on an object is 300 V to 1 kV, and decreases when an energy of the primary electrons incident on an object exceeds 1 kV. Therefore, when the magnitude of the deceleration voltage is set to a value higher than 1 kV, the generating efficiency of the signal secondary electrons on the reflecting plate is decreased and the detection efficiency of the secondary electron from the sample is reduced because the energy of the secondary electrons collide with the reflecting plate exceeds 1 kV. As described above, in the detection method using the reflecting plate described above, there is a problem in that the energy of the electron beam at the time when the electron beam is passing through the objective lens cannot be increased too large because the detection efficiency is decreased when the magnitude of the deceleration voltage is set a value above 1 kV.
Further, regarding the reflected electrons reflected by the sample, the energy after being reflected is nearly the same as the energy when they are incident. For example, when an electron beam of 1 kV is incident on the sample, the energy of the reflected electrons is approximately 1 kV. The reflected electrons are accelerated up to the same voltage as the acceleration voltage of the electron beam by being further accelerated by the deceleration electric field between the objective lens and the sample. Therefore, even when the magnitude of the deceleration electric field is 1 kV, the generating efficiency of the signal secondary electrons on the reflecting plate is decreased because the reflected electrons collide against the reflecting plate with 2 kV. Consequently, the method using the reflecting plate described above has a problem in that the reflected electrons cannot be efficiently detected.